Separation of very close boiling components, such as aromatic and non-aromatic hydrocarbons, is both impractical and uneconomical by conventional distillation. One alternative method of separating close boiling components is extractive distillation (ED). In an ED column, a polar, nonvolatile solvent is introduced into the column near the top to preferentially associate with the more polar components in the feed mixture, so that the relative volatility between the close boiling components can be significantly increased, making the separation possible. A cosolvent may be added to improve solvency or solubility, and to improve overall efficiency of the primary solvent. The relative volatility (xcex1) is a way of expressing the solvent selectivity, and is related to the number of theoretical stages required for the separation. As xcex1 increases, the number of theoretical stages or trays needed to achieve separation decreases. This results in a more commercially viable separation and reduces energy consumption. However, choosing solvent/cosolvent pairs is difficult, and requires actual testing.
The basic principles, design, and operation of ED processes have been thoroughly discussed in the literature, including: Atkins, G. T. et al., xe2x80x9cApplication of McCabe-Thiele Method to Extractive Distillation Calculations,xe2x80x9d Chem. Eng. Prog., 45(9), 553-562 (1949); Chambers, J. M., xe2x80x9cExtractive Distillation Design and Application,xe2x80x9d Chem. Eng. Prog., 47(11), 555-565 (1951); Hackmuth, K. H., xe2x80x9cIndustrial Viewpoints on Separation Processes,xe2x80x9d Chem. Eng. Prog., 48(12), 617-626 (1952); Butler, et al., U.S. Pat. No. 3,114,783; and Perry""s Chemical Engineers"" Handbook, 6th Edition, Mcgraw-Hill Book Company, 1984, pp.13-53 to 13-57. These disclosures are incorporated herein by reference.
Use of extractive distillation to separate aromatics is known, in particular for separating benzene, toluene, and xylene from non-aromatics, where the aromatic and non-aromatic compounds have close boiling points. For example, U.S. Pat. No. 3,591,490 shows a process for separating aromatic hydrocarbons from hydrocarbon mixtures using N-methyl-pyrrolidone or dimethylformamide as a solvent. U.S. Pat. No. 3,723,526 shows a method of recovering aromatic hydrocarbons from a mixture of aromatic and non-aromatic hydrocarbons by a combination of preliminary fractionation, extractive distillation of the fractionation overhead, and solvent extraction of the fractionation bottoms, using sulfolane or other related solvents. U.S. Pat. No. 4,053,369 shows an extractive distillation process that operates with two liquid phases, at an optimized reflux ratio, allowing decreased amounts of solvent to be used. The solvent is chosen to be highly selective, and is preferably a sulfolane-type solvent. Finally, U.S. Pat. No. 4,278,505 shows a process of recovering n-hexane free from aromatic compounds by extractive distillation with a selective solvent such as N-methyl pyrrolidone.
Fu-Ming Lee, xe2x80x9cExtractive Distillation: Close-Boiling-Pointxe2x80x9d Chemical Engineering, 112-120 (1998), describes the use of cosolvents to make difficult separations more economically feasible. This article provides data for the selectivity and solvency of various solvents, as well as their polarity. Solvent/cosolvent pairs tested in the article include cyclohexanol and ethylene glycol, cyclohexanol and tetra ethylene glycol, N-methyl pyrrolidone and ethylene glycol, tetra ethylene glycol and N-methyl pyrrolidone, 3-methyl sulfolane and water, di-n-propyl sulfone and water, and 3-methyl sulfolane and dimethyl sulfone. The article indicates that choosing solvent/cosolvent pairs is difficult due to current limitations on the understanding of the behavior of polar components in solution, so experimentation is necessary to screen cosolvents.
However, none of the above documents teaches the novel solvent and cosolvent combinations that are the subject of the present invention. Accordingly, there is a need to develop more suitable solvents and solvent mixtures than those presently known for use in the ED of mixtures of aromatic and non-aromatic hydrocarbons.
The present invention provides an effective process for separating mixtures of close-boiling aromatics and non-aromatics by extractive distillation using a polar organic solvent or a mixture of polar organic solvents. High purity aromatics may thus be produced from a mixture comprising aromatics and non-aromatics by extractive distillation employing a novel polar organic solvent or a novel mixture of polar organic solvents. Other objects and advantages will be apparent from the detailed description of the invention and the appended claims.
According to one embodiment of the present invention, a process for separating one or more aromatic hydrocarbons from one or more non-aromatic hydrocarbons, in which a feed mixture thereof is subjected to extractive distillation in an extractive distillation column, using sulfolane as extraction solvent, includes the improvement wherein the extraction solvent also includes at least one cosolvent selected from the group consisting of 3-methyl sulfolane, N-methyl-2-pyrrolidone, acetophenone, isophorone, and morpholine.